This invention relates generally to borehole logging apparatus and methods for performing nuclear radiation based measurements. More particularly, this invention relates to a new and improved apparatus for effecting formation density logging in real time using gamma rays wherein the improved nuclear logging apparatus comprises a measurement-while-drilling (MWD) tool.
Oil well logging has been known for many years and provides an oil and gas well driller with information about the particular earth formation being drilled. In conventional oil well logging, after a well has been drilled, a probe known as a sonde is lowered into the borehole and used to determine some characteristic of the formations which the well has traversed. The probe is typically a hermetically sealed steel cylinder which hangs at the end of a long cable which gives mechanical support to the sonde and provides power to the instrumentation inside the sonde. The cable (which is attached to some sort of mobile laboratory at the surface) is also the means by which information is sent up to the surface. It thus becomes possible to measure some parameter of the earth's formations as a function of depth, that is, while the sonde is being pulled uphole. Such "wireline" measurements are normally done in real time (however, these measurements are taken long after the actual drilling has taken place).
A wireline sonde usually contains some type of source (nuclear, acoustic, or electrical) which transmits energy into the formation as well as a suitable receiver for detecting the same energy returning from the formation. The present invention relates to logging apparatus for measuring the density of the formation wherein the source emits nuclear energy, and more particularly gamma rays. Wireline gamma ray density probes are well known and comprise devices incorporating a gamma ray source and a gamma ray detector, shielded from each other to prevent counting of radiation emitted directly from the source. During operation of the probe, gamma rays (or photons) emitted from the source enter the formation to be studied, and interact with the atomic electrons of the material of the formation by photoelectric absorption, by Compton scattering, or by pair production. In photoelectric absorption and pair production phenomena, the particular photons involved in the interacting are removed from the gamma ray beam.
In the Compton scattering process, the involved photon loses some of its energy while changing its original direction of travel, the loss being a function of the scattering angle. Some of the photons emitted from the source into the sample are accordingly scattered toward the detector. Many of these never reach the detector, since their direction is changed by a second Compton scattering, or they are absorbed by the photoelectric absorption process of the pair production process. The scattered photons that reach the detector and interact with it are counted by the electronic equipment associated with the detector.
Examples of prior art wireline density devices are disclosed in U.S. Pat. Nos. 3,202,822, 3,321,625, 3,846,631, 3,858,037, 3,864,569 and 4,628,202. Wireline formation evaluation tools such as the aforementioned gamma ray density tools have many drawbacks and disadvantages including loss of drilling time, the expense and delay involved in tripping the drillstring so as to enable the wireline to be lowered into the borehole and both the build up of a substantial mud cake and invasion of the formation by the drilling fluids during the time period between drilling and taking measurements. An improvement over these prior art techniques is the recently developing art of measurement-while-drilling (MWD) in which many of the characteristics of the formation are determined substantially contemporaneously with the drilling of the borehole. Measurement-while-drilling logging either partly or totally eliminates the necessity of interrupting the drilling operation to remove the drillstring from the hole in order to make the necessary measurements by wireline techniques. In addition to the ability to log the characteristics of the formation through which the drill bit is passing, this information on a real time basis provides substantial safety advantages for the drilling operation. Examples of MWD density devices are disclosed in U.S. Pat. Nos. 4,596,926, 4,814,609 and 4,829,176.
There is a continuing need for new and improved MWD density tools which overcome the limitation associated with both known wireline and MWD density measurement devices. For example, improvements are perceived in achieving more accurate and reliable measurements notwithstanding backscattering of gamma rays through the tool and the presence of drilling fluid (mud) between the tool's detectors and nuclear source and the formation.